The goal of this program is to develop a dedicated, compact accelerator mass spectrometry (AMS) system for the measurement of ultra-low quantities of carbon-14 and tritium in labeled biological samples. The instrument will have a sensitivity at least a factor-of-1000 higher than that of commonly-used liquid scintillation decay counters and will provide high measurement accuracy for the quantitation of sample sizes down to the attomole level. A new approach to AMS will allow the accelerator energy to be lowered by approximately a factor-of-three compared with existing accelerator mass spectrometers. This will greatly reduce system size, cost, and complexity and make possible the development of a laboratory-scale biomedical AMS instrument. The feasibility of this low-energy approach was demonstrated in the Phase I experiments which showed that an AMS system using a 1 MV tandem accelerator will allow the unambiguous detection of 14C at concentrations down to naturally occurring levels in contemporary samples. In Phase II, we propose to construct and test a low-energy AMS system optimized for the detection of 14C and 3H in labeled biological samples. The instrument will utilize a compact, 1 MV tandem accelerator and will be designed to interface with a standard gas chromatograph. PROPOSED COMMERCIAL APPLICATION: Researchers studying carcinogenesis, metabolism and drug effects will benefit greatly from an ultra-sensitive instrument for the quantitation of 14C and tritium in organic samples. In new drug research, the increased sensitivity of AMS will allow human studies to be performed using extremely small quantities of radiotracers. This capability could significantly speed the testing of new drugs. The cost, size, and operating requirements of the instrument will be compatible with installation at major research centers.